Implementation of a large-scale multi-stage non-blocking optical circuit switch

ABSTRACT

Embodiments provide a methodology for designing a large-scale non-blocking OCS using a multi-stage folded CLOS switch architecture for use in datacenter networks and fiber-rich backbone network POPs. One aspect employs a folded CLOS architecture because of its ease of implementation, enabling the topology to scale arbitrarily with increasing number of stages. The fraction of ports allocated for internal switch wiring (overhead) also increases with the number of stages. Design decisions are made to carefully optimize the insertion loss per module, number of ports per module, number of stages and the total scale required. Other embodiments include folded CLOS switch architectures having at least two stages. In one example, power monitoring may be included only on the leaf switches.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/157,964, filed on Jan. 17, 2014, which is a divisional ofU.S. patent application Ser. No. 13/106,384, filed on May 12, 2011, thedisclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to optical interconnect andtransport systems. More particularly, aspects of the invention aredirected to non-blocking optical circuit switching modules.

2. Description of Related Art

Cloud computing and its applications are effecting a qualitative shiftin the way people communicate and share information. The underlyingcomputer networks that support cloud computing can be divided into twomajor categories: intra-datacenter and inter-datacenter. Anintra-datacenter network interconnects the computing infrastructure(e.g., servers, disks) within the same building or among differentbuildings of a datacenter campus. An inter-datacenter network employconnections from metropolitan to long-haul reach interconnectingmultiple datacenters distributed at different geographic locations.Many, if not most modern high-speed data links use optical transmissiontechnologies via optical fibers for both intra- and inter-datacenternetworks.

Currently, most of the actual computing and storage underlying theInternet and cloud computing takes place in warehouse-scale data centerbuildings. Similarly, most of the long-haul links transferring data andrequests back and forth between end users and data centers are switchedthrough Internet points of presence (“POP”). Both environments transfera tremendous amount of data between individual computers and theswitches and routers responsible for getting the data to itsdestination. This bisection bandwidth is often measured in the hundredsof terabits/second in individual data centers and POPs and is expectedto soon surpass the petabit/second mark. Managing this fiberinterconnect can be a significant challenge from a planning anddeployment perspective. Furthermore, re-fibering an existing deployment,for instance to expand capacity, is a very labor-intensive process thatcan be mistake-prone. Consider, for instance, the challenges ofphysically rewiring a fiber plant consisting of multiple tons of short(e.g., less than 300 m) optical fiber patch cords measuring hundreds ofmiles in total length. Given this daunting task, re-fibering may noteven be attempted in many situations.

Fiber infrastructure is typically deployed, organized and interconnectedusing passive fiber patch panels. These patch panels are made of arraysof passive fiber mating connectors. Therefore, network topologies areoften built in a manual and static fashion. Often, there is neitheractive power monitoring for fault detection nor troubleshooting, nor isthere a capability to automatically protect against failures. Changerequests to the patch panel connectivity, e.g., to expand bandwidthcapacity or to recover from connection failures, would require localon-site access and manual rewiring. This manual, labor intensive processcan stretch across multiple days because of access approval and travelto remote locations, thus leading to long mean-time-to-repair (“MTTR”)and network performance degradation.

Optical circuit switching (“OCS”) has been one approach to address theabove issues. OCS serves as a non-blocking active patch panel that canbe controlled remotely and programmed to set up connections between anyports. It is typically implemented through mechanical switchingmechanism and direct light beams between different ports either in freespace or through physical connections. However, such OCS architecturesmay have a limited number of ports or may have slow switching speeds.

SUMMARY OF THE INVENTION

Aspects of the invention present a design and implementation of alarge-scale non-blocking optical circuit switch using multipleindividual OCS modules to address applications for fiber-rich facilitiessuch as datacenters and high-degree backbone POPs. The individual OCSmodules may be constructed via either monolithic or multi-chip module(“MCM”) techniques.

An optical circuit switching device according to one embodimentcomprises a first stage including a first plurality of 2n by 2n opticalcircuit switching modules, a second stage including a second pluralityof 2n by 2n optical circuit switching modules, and a plurality ofoptical fibers. The second plurality of optical circuit switchingmodules is fewer than the first plurality of optical circuit switchingmodules, and the plurality of optical fibers interconnects ports of thefirst and second pluralities of optical circuit switching modules in atwo-stage folded CLOS topology.

In one example, each optical circuit switching module is a non-blockingswitching module. In another example, the plurality of optical fibers isfusion spliced to the ports for propagating optical signals between thefirst and second stages. In a further example, the device also comprisesa front plate coupled to the second stage via a second plurality ofoptical fibers. The front plate is configured to interface with one ormore external devices.

In another example, the device further comprises a system control planeoperatively coupled to the first stage, the second stage and the frontplate. Here, the system control plane is configured to control andmonitor overall operation of the optical circuit switching device. Inyet another example, each optical circuit switching module of the firststage includes a power monitoring mechanism configured to performend-to-end power monitoring. In one alternative, none of the opticalcircuit switching modules of the second stage includes a powermonitoring mechanism.

In a further example, the optical circuit switching modules of the firstand second stages comprise monolithic MEMS devices. In another example,the first plurality has 2n optical circuit switching modules and thesecond stage has n optical circuit switching modules. In yet anotherexample, the two-stage folded CLOS topology is oversubscribed. In thiscase, multiple uplink ports from a given one of the optical circuitswitching modules of the first stage may be connected to a selected oneof the optical circuit switching modules of the second stage.

And in another example, the optical circuit switching device furthercomprises a third stage including a third plurality of 2n² opticalcircuit switching modules. Each module in the third plurality has 2nport switches. In this example, the first stage comprises a plurality offirst stages, the second stage comprises a plurality of second stages,the plurality of first and second stages form multiple layers, and theswitches of the modules in the third stage interconnect the multiplelayers.

In accordance with another embodiment, a method of designing an opticalcircuit switching device comprises determining a target number of portsin a set of optical circuit switching modules of the optical circuitswitching device; identifying an insertion loss distribution across allconnections for the set of optical circuit switching modules; performinga fine-grained, automated link budget analysis to ensure thatend-to-end, high insertion loss connections are matable to lowerinsertion loss connections to stay within an end-to-end optical budget;and configuring a multi-stage folded CLOS topology with the set ofoptical circuit switching modules to minimize overhead given the targetnumber of ports and the insertion loss distribution.

In one example, the method further comprises identifying a scalingrequirement for the CLOS topology. If the identified insertion lossdistribution and scaling requirement cannot be satisfied simultaneouslyby a two-stage folded CLOS topology, then in one alternative configuringthe topology includes selecting optical circuit switching modules havingfewer than the target number of ports. In another example, the set ofoptical circuit switching modules includes a first group and a secondgroup, and configuring the topology includes selecting only the opticalcircuit switching modules of the first group to include a powermonitoring mechanism.

According to another embodiment, a method for implementing a multi-stageCLOS optical circuit switch architecture comprises choosing a singlemodule having a port count n and a connection insertion loss α;determining, with a processing device, whether a scaling requirement fora total number of ports) is satisfied; determining, with the processingdevice, whether an insertion loss requirement is satisfied; and whenboth the scaling requirement and insertion loss requirement aresatisfied, configuring a two-stage CLOS optical circuit switcharchitecture according to the port count n and the connection insertionloss α.

If the scaling requirement is satisfied and the insertion lossrequirement is not satisfied, then according to one example the methodfurther comprises modifying the port count to be m, where m is less thanor equal to n; and modifying the insertion loss to be β, where β is lessthan α. If the scaling requirement is not satisfied, then according toanother example the method further comprises configuring a three-stageCLOS design. Here, in one alternative, the method further comprisesmodifying the port count to be m, where m is greater than n.

Alternatively, if the scaling requirement is satisfied and the insertionloss requirement is not satisfied, the method further comprisesmodifying the port count to be m, where m is less than or equal to n,and modifying the insertion loss to be β, where β is less than α.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a CLOS network for use with embodiments of theinvention.

FIG. 2 illustrates a non-blocking CLOS network for use with embodimentsof the invention.

FIGS. 3( a)-(c) illustrate development of a CLOS-type topology for usewith aspects of the invention.

FIG. 4 illustrates a 3-stage CLOS topology in accordance with aspects ofthe invention.

FIG. 5 illustrates a 2-stage folded CLOS optical circuit switch inaccordance with aspects of the invention.

FIG. 6 illustrates a process for implementing a multi-stage CLOS opticalcircuit switch.

DETAILED DESCRIPTION

The aspects, features and advantages of the present invention will beappreciated when considered with reference to the following descriptionof embodiments and accompanying figures. The same reference numbers indifferent drawings may identify the same or similar elements.Furthermore, the following description does not limit the presentinvention; rather, the scope of the invention is defined by the appendedclaims and equivalents.

One approach to implementing a large-scale non-blocking switch fromavailable low radix non-blocking switches is the CLOS topology. CLOSnetworks are used to construct a larger-scale switch when theswitching-port needs exceed the radix of the largest available singleswitch. Historically, the CLOS topology approach has been applied toelectrical switching environments for real-time and interactive datadelivery, as it was originally designed to solve capacity constraints intelephone switching networks. The CLOS topology has since been appliedto electronic packet switching environments for real-time andinteractive data delivery among multiple integrated circuit (IC) chips.

A CLOS network is a multistage switching network first conceived byCharles Clos in 1953, representing a theoretical idealization ofpractical multi-stage telephone switching systems. FIG. 1 illustrates aCLOS network topology for use with aspects of the invention. The inputstage consists of k switches with n ingress ports and m egress ports (ann×m switch). The middle stage consists of m k×k switches. And the outputstage consists of k m×n switches.

FIG. 2 illustrates a special case, where m=n and k=2n. In this case, theCLOS network is strictly non-blocking if each individual switch isnon-blocking, which means that an unused ingress port on the left canalways find a free path to an unused egress port on the right withouthaving to re-arrange any of the existing connections.

The CLOS topology may be modified by folding the flat two-dimensionalstructure along the center of the middle stage. In this case, everyingress port has a matched egress port due to the symmetry of thetopology, which is shown in FIGS. 3( a) and (b). Here, m=n and k=2n, andall switches become identical with 2n duplex ports. Each pair of matchedingresses and egresses may be referred to as a duplex port. As usedherein, the term “port” refers to a duplex port unless otherwise stated.By turning the topology 90 degrees, it becomes a 2-stage folded CLOS or“FAT TREE” network as shown in FIG. 3( c). The result is a 2-stagefolded CLOS topology built with identical 2n-port switches to itsmaximum scale. The top stage is referred to as “spine” switches, and thebottom stage is referred to as “leaf” switches.

The 2-stage folded architecture of FIG. 3( c) has several advantagesover the original CLOS topology. For example, it constructs alarge-scale non-blocking topology by using switching elements of thesame type, such as identical switching elements. In this case, eachswitch has 2n ports (it is a 2n×2n non-blocking switch). In addition,the total number of stages and total number of switches are reduced.Furthermore, it is easier to scale up even further from a 2-stage foldedCLOS to a 3-stage folded CLOS. FIG. 4 shows an example of a 3-stage CLOStopology deploying 2n-port switches. As shown, the 2-stage topology isfirst repeated n times, and then 2n² third stage leaf switches are addedto interconnect these n layers of 2-stage networks.

The ultimate scaling limit of a 2-stage folded CLOS using 2n-portswitches is 2n² ports, if a complete fan-out from any one of the nuplink ports of a leaf switch is connected to a unique spine switch. Asmaller scale and oversubscribed topology may be realized by connectingm (where m<n) uplink ports from one leaf switch to the same spineswitch. Here, the total number of spine switches becomes n/m and thereis an oversubscription ratio of m:n. Therefore, the total number ofports of the CLOS topology becomes 2n²/m ports. Similarly, for a 3-stageexample as shown in FIG. 4, the maximum port count is 2n³.

However, it is recognized that there are a large number of ports thatare used only for internal wiring among switches in different stages.The ratio between external ports and total ports may be used as aparameter to measure how effective and efficient the topology is interms of cost per external port. For instance, in a 2-stage design,2n²/6n²=⅓ of the total ports are external user facing ports, resultingin 67% overhead of ports for internal wiring. And in a 3-stage design,2n³/10n³=⅕ of the total ports are externally useful ports, with 80% ofwasted ports used for internal wiring only. Therefore, moving up withmore stages, although increasing the scale of CLOS topology, wouldincrease the per-port cost of the CLOS topology.

Conventional single-module optical circuit switches are typically basedon one of several general technologies. The most common is themicro-electromechanical system (“MEMS”) technology, in which arrays of2-dimensional silicon mirrors are used to steer optical beams in freespace between different ports. Another approach uses piezoelectricactuators to steer optical beams in free space between different ports.A third technique is based on a dynamic multi-layer optical couplingtechnology that leverages high-precision motors driving two sets offibers to couple the light through matched ports. All three technologieshave scaling limits in terms of port count. MEMS based technology islimited by the silicon chip yield and optical coupling loss of thecollimating lens. Piezoelectric technology is limited by the smallswitching angle and physical space. High-precision motor basedtechnology is limited by the physical space consumed by the dynamicmulti-layer optical coupling design, and it also requires orders ofmagnitude longer switching time to set up a physical connection.

As discussed above, a 2-stage folded CLOS topology can be an effectiveapproach to scale up the port count. In this section, design andimplementation details are provided for applying this topology in theoptical domain to come up with a viable large-scale OCS solution. Thisincludes an interconnectivity topology to address internal wiringoverhead, fiber management and connectivity issues at large scale,optical insertion loss and power budget for link design, insertion lossand scaling optimization strategies, and built-in power monitoringschemes to manage end-to-end connectivity.

In order to obtain a large-scale non-blocking OCS, one aspect of theinvention leverages existing small-radix OCS modules to construct amulti-stage folded CLOS with a shared electrical control plane. To meetthe datacenter scaling requirement while maintaining reasonable per-portcost at the same time without too much internal wiring overhead, a2-stage topology may be employed using a single OCS with relativelylarge port count. Individual OCS modules are not limited to a particulartechnology as long as the system meets the port count requirement andcan be packaged into a footprint of a single chassis or a single rackfor further scaling. However, this does not rule out the possibility ofbuilding a topology with more than two stages to scale up further tomeet a range of application requirements.

According to one embodiment, each OCS module includes fiber attached toeach simplex port. The ports used for internal connectivity between theleaf and spine switch modules in two stages may be connected through alow-loss method such as fusion splicing. These internal connections mayalso be realized by terminating all fibers with matched connectors.However, there are several advantages of fusion splicing over thematched connector approach. First, fusion splicing incurs a very smalloptical loss. Second, it typically has a lower cost since no connectorsand mating couplers are needed. And third, because the internal wiringpattern is fixed once the scale of the CLOS network is determined,fusion splicing on large amount of fibers can be done automatically witha computer-aided process instead of a costly labor-intensive manualprocess with frequent human errors. On the other hand, the externaluser-facing ports may be terminated with fiber connectors to be mountedon the front plate. For a large scale design, high-density fiberterminations such as (but not limited to) MTP (Mechanical transfer pushon) or MPO (multi-fiber push on)-type connectors, may be used, as shownin the 2-stage folded CLOS OCS architecture 500 of FIG. 5.

Here, the spine stage includes n 2n×2n OCS modules 502. The leaf stageincludes 2n 2n×2n OCS modules 504. The OCS modules 502 and 504 areinterconnected using optical fibers, where each line 506 represents twofibers for a duplex port. The leaf OCS modules 504 are connected to afront plate 508, which includes high density fiber connectors 510 forinterconnection to other devices (not shown). System control plane 512connects all modules and controls the overall operation of the entiresystem. The system control plane 512 may be composed of off-the-shelf ICchips such as a microprocessor, memory, auxiliary communications ports,etc. and may follow standard digital system design to realize thedesired control functions.

One exemplary embodiment of this architecture employs monolithicMEMS-based optical circuit switches with good port counts of sufficientsize to construct the 2-stage folded CLOS. The maximum port counts perMEMS OCS module is often limited by the yield of MEMS silicon and scaleof OCS module packaging. In such a configuration, each MEMS OCS modulehas fiber connections 506 to all ports of all other OCS modules. Theports used for propagating a signal from the first stage to the secondstage may be physically connected using a very low loss method, e.g.,fusion splicing, while the ports facing end users may be terminated withthe high-density fiber connectors 510.

Unlike electrical packet switching in which optical-electrical-optical(“O-E-O”) conversion occurs at each switch interface, optical circuitswitching is straight cut-through with native optical propagation alongthe entire path. Therefore, it is important to minimize the opticalinsertion loss going through each OCS so that cost-effective opticaltransceivers with limited optical power budget can be used to drivenetwork links. The worst case optical switching path from any port toany port is through three different switches (two leaf switches and onespine switch) as indicated in FIG. 5 by thick lines 514 as an example.Therefore, the maximum insertion loss for a 2-stage folded CLOS OCS isthree times of the maximum insertion loss of an individual OCS plusfusion splicing loss. OCS technologies that involve free-space opticsusually would result in higher insertion loss with increasing port countbecause of beam divergence associated with longer optical paths.However, a higher port count (n) per OCS module leads to larger scalinglimit in a 2-stage topology (following 2n²).

When insertion loss and scaling requirement cannot be satisfiedsimultaneously by a 2-stage design, an alternative approach would todeploy OCS modules with smaller port counts but with a lower maximuminsertion loss per stage to build higher order CLOS topology, such asthree stages or more, to meet the scaling requirement. Using morestages, the overall insertion loss may still be lower as long as theinsertion loss reduction per module with smaller port count surpassesthe worse-case hop increment due to more stages. In addition, as long asthe OCS module port count used in a 3-stage is, for example, greaterthan x³ (where x is the port count reduction ratio compared to themodule used in a 2-stage) the total scale of a 3-stage topology isgreater than a 2-stage topology. Therefore, a good design decisionshould be made by carefully optimizing the insertion loss per module,number of ports per module, number of stages and the total scalerequired.

An interesting property of OCS modules is that the insertion loss varieswith the free-space path between the input port and the output portfollowing a certain statistical distribution. The worst-case insertionloss must be reported for the specification, although the typicalinsertion loss value may be substantially lower. Hence, manufacturersneed to carefully balance the specification with yield, cost and themaximum number of ports. Currently, this tradeoff is typically madeassuming a single-stage deployment.

To enable scaling to multi-stage scenarios with a stringent optical linkbudget requirement, one aspect of the invention employs the followingstrategy to work with commodity OCS modules designed for single-stageuse. If a target number of ports in a deployment is known, one can comeup with different solutions of different CLOS stages built with OCSmodules having different port counts. Once the insertion lossdistribution across all ports of the available OCS modules are known,one can decide on a solution by choosing the maximum number of portsfrom an individual module capable of supporting an end-to-end insertionloss across the multi-stage topology while still achieving the scalingtarget. For example, the final test data for insertion loss availablefor each of the n×n connections may be made available via the switchcommand API. This allows the network management software to be aware ofinsertion loss variations within each switch.

Consider a simple scenario involving a 10,000-port switch with anend-to-end optical link budget of 6 dB (not including the internalwiring loss), having a worst-case insertion loss for a given 320-portOCS module of 3 dB. This would preclude deploying this 320-port modulein a 2-stage CLOS topology with all random internal connections becausethe worst-case path would follow 3 hops with a worst-case totalinsertion loss of 9 dB. However, if one could select only 160 ports inthe module with an insertion loss of less than 2 dB, a 2-stage CLOS maystill be built while satisfying the end-to-end link budget requirementsin this scenario. Note that this involves a tradeoff eliminating 50% ofthe ports with higher loss to enable a 2-stage design capable of scalingup to 160×80=12,800 ports, a significant capability beyond the reach ofthe original 320-port modules.

FIG. 6 illustrates a process 600 for implementing a multi-stage CLOSoptical circuit switch according to one aspect of the disclosure. Asshown, the design may begin at block 602 with a single module having aport count n and a connection insertion loss α. At block 604, thetwo-stage design is begun. It is determined at block 606 whether thescaling requirement (e.g., the total number of ports) is satisfied. Ifthe scaling requirement is satisfied, the process proceeds to block 608,where it is determined whether the insertion loss requirement issatisfied. If this condition is true, then the process ends at block610. If the insertion loss condition is not true, then the processreturns to block 602, while modifying the port count to be m, where m isless than or equal to n, and modifying the insertion loss to be β, whereβ is less than α.

If the scaling requirement in block 606 is not satisfied, then theprocess proceeds to block 612 for a three-stage design. At block 614 itis determined whether the scaling requirement (e.g., the total number ofports) is satisfied. If the scaling requirement is satisfied, theprocess proceeds to block 616, where it is determined whether theinsertion loss requirement is satisfied. If the insertion loss conditionis true, then the process ends at block 618. If this condition is nottrue, then the process returns to block 602, while modifying the portcount to be m, where m is less than or equal to n, and modifying theinsertion loss to be β, where β is less than α. If the scalingrequirement condition in block 614 is not satisfied, then the processreturns to block 602 to start with a single module having port count m,but where m is greater than n.

To address port to port reliability concerns, a power monitoringmechanism may be employed for fault detection and protection. Powermonitoring may be included on each switching module in one example;however, in another example, instead of adding power monitoring on everyswitch module, it is sufficient to have power monitoring on leafswitches only, to perform end-to-end monitoring to minimize totaloptical insertion loss and system cost. The power monitoring may be doneusing a waveguide coupler or splitter to tap a smaller percentage ofoptical power.

As discussed above, instead of constructing a flat 2-dimensional CLOSnetwork, one aspect of the invention employs a folded CLOS architecturebecause of its ease of implementation. This enables the topology toscale arbitrarily with increasing number of stages. However, thefraction of ports that must be allocated for internal switch wiring(overhead) also goes up with the number of stages. For example, a2-stage design makes ⅓ of the total ports available as user facingports, with the remaining ⅔ making up overhead in the form of internalswitch wiring. The amount of overhead for a 3-stage CLOS topology goesup to 80%. Higher overhead of course results in higher overall cost fora CLOS topology. Hence, a particular network design should employ onlyas many stages as necessary to achieve its scaling target.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. An optical circuit switching device,comprising: a first stage including a first plurality of 2n by 2noptical circuit switching modules; a second stage including a secondplurality of 2n by 2n optical circuit switching modules, the secondplurality of optical circuit switching modules being fewer than thefirst plurality of optical circuit switching modules; a first pluralityof optical fibers interconnecting ports of the first and secondpluralities of optical circuit switching modules in a two-stage foldedCLOS topology; wherein each optical circuit switching module of thefirst stage includes a power monitoring mechanism configured to performend-to-end power monitoring across at least one optical circuitswitching module of the first stage and at least one optical circuitswitching module of the second stage; and wherein none of the opticalcircuit switching modules of the second stage includes a powermonitoring mechanism.
 2. The optical circuit switching device of claim1, wherein each optical circuit switching module is a non-blockingswitching module.
 3. The optical circuit switching device of claim 1,wherein the plurality of optical fibers is fusion spliced to the portsfor propagating optical signals between the first and second stages. 4.The optical circuit switching device of claim 3, further comprising afront plate coupled to the first stage via a second plurality of opticalfibers, the front plate being configured to interface with one or moreexternal devices.
 5. The optical circuit switching device of claim 4,further comprising a system control plane operatively coupled to thefirst stage, the second stage and the front plate, the system controlplane being configured to control and monitor overall operation of theoptical circuit switching device.
 6. The optical circuit switchingdevice of claim 1, wherein the optical circuit switching modules of thefirst and second stages comprise monolithic MEMS devices.
 7. The opticalcircuit switching device of claim 1, wherein the first plurality has 2noptical circuit switching modules and the second stage has n opticalcircuit switching modules.
 8. The optical circuit switching device ofclaim 1, wherein the two-stage folded CLOS topology is oversubscribed.9. The optical circuit switching device of claim 8, wherein multipleuplink ports from a given one of the optical circuit switching modulesof the first stage are connected to a selected one of the opticalcircuit switching modules of the second stage.
 10. The optical circuitswitching device of claim 1, wherein the power monitoring mechanism inthe first stage is configured to minimize an optical insertion loss ofeach optical circuit switching module in the second stage.
 11. Anoptical circuit switching device, comprising: a first stage including afirst plurality of optical circuit switching modules; a second stageincluding a second plurality of optical circuit switching modules, thesecond plurality of optical circuit switching modules being fewer thanthe first plurality of optical circuit switching modules; and aplurality of optical fibers interconnecting ports of the first andsecond pluralities of optical circuit switching modules in a two-stagefolded CLOS topology; wherein each optical circuit switching module ofthe first stage includes a power monitoring mechanism configured toperform end-to-end power monitoring across at least one optical circuitswitching module of the first stage and at least one optical circuitswitching module of the second stage.
 12. The optical circuit switchingdevice of claim 11, wherein each module in the first plurality has 2nports and each module in the second plurality has 2n ports.
 13. Theoptical circuit switching device of claim 11, further comprising a thirdstage including a third plurality of 2n² optical circuit switchingmodules.
 14. The optical circuit switching device of claim 13, whereineach module in the third plurality has 2n ports.
 15. The optical circuitswitching device of claim 13, further comprising: a plurality of firststages; and a plurality of second stages; wherein the plurality of firstand second stages form multiple layers; and wherein the modules in thethird stage interconnect the multiple layers.
 16. An optical circuitswitching device, comprising: a first stage including a first pluralityof optical circuit switching modules; a second stage including a secondplurality of optical circuit switching modules, the second plurality ofoptical circuit switching modules being fewer than the first pluralityof optical circuit switching modules; a plurality of optical fibersinterconnecting ports of the first and second pluralities of opticalcircuit switching modules in a two-stage folded CLOS topology; and atleast one power monitoring mechanism configured to perform end-to-endpower monitoring across at least one optical circuit switching module ofthe first stage and at least one optical circuit switching module of thesecond stage, the at least one power monitoring mechanism included onlyon each of the first stage optical circuit switching modules.